The present invention relates to radiation detectors such as those used in computed tomography imaging systems; and particularly to techniques which compensate for offset and afterglow errors in output signals from the detectors.
As shown in FIG. 1, a computed tomography (CT) scanner for producing images of the human anatomy includes a motorized patient table 10 which positions a patient at different depths within aperture 11 of a gantry 12. A source of highly collimated X-rays 13 is mounted within the gantry 12 to one side of its aperture 11, and one or more detectors 14 is mounted to the other side of the aperture. The X-ray source 13 and detectors 14 revolve about aperture 11 during a scan of the patient to obtain X-ray attenuation measurements from many different angles.
A complete scan of the patient is comprised of a set of X-ray attenuation measurements which are made at different angular orientations of the X-ray source 13 and detector 14. The gantry may stop or continue to move as the measurements are being made. The measurement at a given orientation is referred to in the art as a "view" and the set of measurements at a view forms a transmission profile. As shown in FIG. 2, the X-ray source 13 produces a fan-shaped beam which passes through the patient and impinges on an array of detectors 14. Each detector 14 in this array produces a separate attenuation signal and the signals from all the detectors 14 are separately acquired to produce the transmission profile for the indicated angular orientation. The X-ray source 13 and detector array 14 continue to revolve in direction 15 to a another angular orientation where the next transmission profile is acquired.
The resultant transmission profiles from the scan then are used to reconstruct an image which reveals the anatomical structures in a slice taken through the patient. The prevailing method for reconstructing image is referred to in the art as the filtered backprojection technique. The attenuation measurements are converted to integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a CRT display.
Typically a number of scans are taken as the table 10 moves into aperture 11 to produce image slices through the patient at different locations. It is desirable to minimize the period between scans to reduce the overall time of the patient examination. Presently available systems have a one second inter-scan period. However the response of the radiation detector 14 is not instantaneous and such short inter-scan periods result in the detector retaining residual effects from the previous scan when the next scan begins.
Specifically, each X-ray detector 14 in the array comprises a scintillator and a solid state photodiode. X-rays striking the scintillator produce light photons which are absorbed by the photodiode creating an electric current. The light is not emitted by the scintillators instantaneously, rather the emission follows a multi-exponential curve. Similarly the light emission does not terminate immediately at time T.sub.0 when the X-ray beam is extinguished, but produces a signal from the detector which decays with a multi-exponential curve function as shown in FIG. 3. The time dependence of the output signal intensity can be modelled accurately as a sum of several different time constants components. The shortest of the decay components has a time constant on the order of one millisecond and is referred to herein as the "primary speed". The primary speed component accounts for most of the residual output from the detector after the X-ray beam is extinguished. However, the remaining components can have time constants as long as several hundred milliseconds.
Because the detector array is rotating rapidly about the patient, the exponential decay blurs together detector readings for successive views creating an adverse effect that is referred to as "afterglow". The afterglow is a function of the intensity of the X-ray flux and the response characteristics of the detector. U.S. Pat. No. 5,249,123 entitled "Compensation of Computed Tomography Data for Detector Afterglow Artifacts", filed concurrently herewith, describes a technique which can be employed to compensate for afterglow effects during each scan. However, when a relatively short amount of time exists between consecutive scans, the afterglow from the previous scan affects the detector signal for the subsequent scan. The afterglow degrades the azimuthal component of the image resolution which produces shading and arc shaped artifacts in the reconstructed image. The azimuthal direction 16 of the image area is perpendicular to a line 17 from the center of the imaging aperture 11. Thus it is desireable to compensate for afterglow effects from one scan to the next.
Another error is an offset A.sub.0 of the detector signal in FIG. 3 due to an number of factors, such as the dark current of the detector and thermal drift. Previously, the dark current was measured prior to each scan, when radiation is not striking the detector, and the measurement was used to derive a dark current compensation value. This compensation value is applied to subsequent detector signals to remove the offset due to the dark current. Thermal and time dependent factors also contribute to the overall offset error and vary the offset from scan to scan.
The dynamic offset components for each detector in the array can be measured between scans to generate compensation values. However, the measurement heretofore could not be made until sufficient time had elapsed after the beam extinguished to allow the afterglow to decay to a negligible level. The afterglow decay can last 600 milliseconds or more. In addition, the X-ray source 13 has a non-instantaneous response producing a delay between the shut-off of the source and the extinction of the X-ray beam. Taking into account all of these factors a three second waiting period is required after a scan before offset data acquisition can start. As the operating speed of CT systems increase, even a three second delay prolongs image acquisition and makes the process more susceptible to errors from patient movement. As a consequence it is desirable to be able to acquire data for offset compensation without having to wait for the afterglow to decay to a negligible level.